The present invention pertains to an improvement in a circulator for a stripline microwave transmission line circuit.
A circulator is a microwave coupling device having three or more terminals, and is so constructed that energy entering one terminal is transmitted to the next adjacent terminal in a particular direction. Although circulators have been used for many years their principal application has been in coupling microwave energy transmitted in waveguides. Recently there has been a demand for circulators for coupling microwave energy transmitted in stripline devices, thereby necessitating a drastic reduction in the size and weight of the circulator.
One circulator that was designed for use in a stripline configuration is described in U.S. Pat. No. 3,334,317 to Andre. As described therein, the stripline microwave transmission line circuit is disposed in a pattern of printed circuit conductors between a pair of facing dielectric material circuit boards having conductive ground planes on their exposed broad faces. The circulator includes three printed circuit center conductors disposed between the two boards in a symmetrical array extending radially from a circular hole aligned through the two boards, wherein the center conductors and the ground planes define transmission lines terminating in ports at the circular hole; a conductive foil having a central portion concentrically disposed within the perimeter of the hole and three radially projecting arms arranged in contact with the radially extending center conductors for electrically connecting the center conductors together to provide signal paths between adjacent ports; a pair of pucks of ferrite material of a predetermined size selected to provide circulation of microwave signals of a predetermined wavelength concentrically disposed in the hole on opposite sides of the conductive foil; a pair of conductive covers in contact with the ground plane conductors to provide an rf ground for the ferrite pucks, and extending to completely enclose the ferrite pucks within the hole; a pair of magnets disposed on opposite sides of the boards in axial alignment with the ferrite pucks for providing a magnetic field through and normal to the central portion of the conductive foil to thereby enable a microwave signal to be circulated from a transmission line port defined by one center conductor and its respective ground planes to a port defined by an adjacent center conductor and its ground planes, in a direction determined by the direction of the magnetic field; and magnetically permeable material contacting the magnets and concentrically disposed for providing a closed magnetic loop for shielding the circulator.
Magnetic shielding of the circulator is quite important. Because circulators include one or two disc magnets to provide magnetic bias, if no magnetically permeable means for directing the return path is included, the return path for the magnetic field is through the surrounding region. If there is a magnetic material such as Kovar near an exposed magnet face, or a second unshielded circulator nearby, the presence of the ferromagnetic material or the second circulator will change the field in and de-tune the first circulator.
At the same time as circulators were being made smaller, signal sources for microwave transmission systems were growing more complex. In some pulsed coherent sources, a number of Y-junction circulators are needed. Proliferation of Y-junction circulators in a single subsystem means a larger fraction of the system's weight and volume is devoted to the circulators. Thus, a small light-weight circulator which can be made an integral part of a stripline circuit is now almost mandatory.
Also, consideration must be given to the fact that as the components of the circulator become smaller their relative dimensions become much more critical in view of the microwave frequencies at which the circulators are used. In this regard it is particularly important to provide a circulator which can readily be impedance-matched to the transmission lines defined by the center conductors at the predetermined frequency (or wavelength) of operation for which the system is designed.